Regulations of the United States Federal Communications Commission (FCC) specify that the maximum horizontal blanking width permitted to be broadcasted is 11.44 microseconds at that level of the radiated signal corresponding to 90 IRE units. Television plant signal processing techniques often result in horizontal blanking widths considerably in excess of this, and additional blanking errors are sometimes introduced by misadjustments and equipment malfunctions. It has been found that some video signals exceed the FCC horizontal blanking requirements by as much as two microseconds.
Production houses are often to blame for supplying video taped material with incorrect blanking. Whatever the reasons, most television stations are saddled with programs that do not meet the blanking specifications of the FCC. Although stricter quality control can prevent excessive blanking in future productions, there remains a need to correct excessive blanking on older material and in those cases where normal studio processing or undetected equipment failure results in excessive blanking. Since the FCC appears to be taking a posture of stricter enforcement of its horizontal blanking specifications, there is a need for a horizontal blanking width corrector for NTSC color television signals.
Since the excessive horizontal blanking normally introduced by plant operations is usually of the order of a few hundred nanoseconds, it can be corrected by stretching the active video out into the blanked picture area. This process introduces some excess picture width, but it is not objectionably noticeable; under worst case conditions where two microseconds of excessive blanking is corrected, 4% horizontal non-linearity will be introduced. This makes the aspect ratio of the picture greater than the specified 4 to 3, but it should be subjectively acceptable provided it is not compared side-by-side with an unstretched picture. The process of expansion can be readily performed by changing the scanning time base. For example, a digital time base corrector samples and stores video into a memory at a sampling frequency F.sub.s and then reads the video out of memory at a different rate (F.sub.s -.DELTA.). More specifically, to expand an NTSC color television picture, one could sample the signal at four times the color subcarrier rate (4F.sub.sc), write the signal into a memory, and then read it out of memory at a (4F.sub. sc -.DELTA.) rate. The resultant picture will undergo a (.DELTA./4-F.sub.sc).times.100% horizontal expansion. This type of expansion, however, shifts all signal frequencies by (.DELTA./4-F.sub.sc).times.100%, resulting in an incorrect color subcarrier.
The primary object of the present invention is to provide a blanking width corrector system wherein picture width is expanded while maintaining the integrity of the color subcarrier frequency.